Lithium secondary battery and electric device

Abstract

A lithium secondary battery and an electric device, relating to the technical field of lithium batteries. A positive electrode active material of the lithium secondary battery comprises lithium-containing phosphate particles, some of which have a particle size greater than or equal to 1 μm; in a lithium-containing phosphate, the number percentage of particles having a particle size greater than or equal to 5 μm and less than or equal to 15 μm is greater than 0 and less than or equal to 3%; in the lithium-containing phosphate, the number percentage of particles having a particle size greater than or equal to 200 nm and less than or equal to 1 μm is greater than or equal to 50% and less than or equal to 75%; and in the lithium-containing phosphate, the number percentage of particles having a particle size greater than or equal to 50 nm and less than 200 nm is greater than or equal to 25% and less than 50%. By controlling the proportions of the three types of lithium-containing phosphate particles having different particle sizes in the lithium-containing phosphate to fall within the ranges described above, a better particle gradation effect can be achieved, thereby increasing the tap density and compaction density of the lithium-containing phosphate, and thus improving the energy density of a lithium secondary battery comprising the lithium-containing phosphate.

Description

A lithium secondary battery and power device

[0001] Cross-references to related applications

[0002] This application claims priority to Chinese Patent Application No. 202510368616.2, filed on March 26, 2025, entitled “A Lithium Secondary Battery and an Electrical Device”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of lithium battery technology, and in particular to a lithium secondary battery and an electrical device thereof. Background Technology

[0004] In recent years, with the increasingly widespread application of lithium-ion batteries, they have been widely used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and many other fields. Due to the significant advancements in lithium-ion battery technology, higher requirements have been placed on their energy density, cycle performance, and safety performance. Furthermore, due to the increasingly limited selection of positive electrode active materials, lithium phosphate-containing positive electrode materials are considered the best choice for meeting safety requirements.

[0005] However, the relatively low energy density of lithium phosphate cathode materials limits their application in some high-performance scenarios. Summary of the Invention

[0006] This application is made in view of the above-mentioned issues, and its purpose is to provide a lithium secondary battery and an electrical device that can improve the energy density of lithium phosphate cathode materials.

[0007] To achieve the above objectives, a first aspect of this application provides a lithium secondary battery, comprising a positive electrode sheet, the positive electrode sheet comprising a positive electrode active material, the positive electrode active material comprising lithium phosphate, wherein the proportion of lithium phosphate particles with a particle size ≥ 1 μm in the positive electrode active material is ≥ 0.2%. The proportion of lithium phosphate particles with a particle size ≥ 5 μm and ≤ 15 μm is ≥ 0 and ≤ 3%, the proportion of lithium phosphate particles with a particle size ≥ 200 nm and ≤ 1 μm is ≥ 50% and ≤ 75%, and the proportion of lithium phosphate particles with a particle size ≥ 50 nm and ≤ 200 nm is ≥ 25% and ≤ 50%.

[0008] Therefore, the positive electrode active material of the lithium secondary battery in this application includes some lithium phosphate particles with a particle size ≥1μm. However, lithium phosphate particles with a particle size ≥1μm not only have low packing density, making it difficult to improve compaction density, but also have a longer diffusion path for lithium ions due to their larger size, resulting in a slower diffusion rate. Consequently, during charging and discharging, lithium ions cannot be quickly inserted / extracted, leading to a decrease in capacity utilization and energy density. This application, by controlling the ratio of three different particle sizes of lithium phosphate particles within the aforementioned range, enables better gradation among the three different particle sizes of lithium phosphate particles. They can cooperate and fill each other, thereby improving the tap density and compaction density of the lithium phosphate, and thus improving the energy density of the lithium secondary battery containing lithium phosphate.

[0009] In any embodiment, the proportion of lithium phosphate particles with a particle size ≥1μm in the positive electrode active material is 0.2% to 2%. This application achieves this by keeping the proportion of lithium phosphate particles with a particle size ≥1μm within the above range. Since lithium phosphate particles with a particle size ≥1μm have poor conductivity, they exhibit greater polarization. During the early cycling stages, not all lithium ions can return to the negative electrode after deintercalation. As cycling progresses, the lattice of the lithium phosphate particles with a particle size ≥1μm undergoes repeated expansion and contraction, leading to particle breakage and the exposure of more interfaces. This improves the conductivity of the lithium phosphate particles with a particle size ≥1μm, reduces polarization, and allows more anions to flow back from the negative electrode to the positive electrode, thereby improving the problem of rapid degradation of the early cycle life of lithium secondary batteries containing lithium phosphate.

[0010] In any embodiment, the proportion of particles with a diameter greater than or equal to 5 μm and less than or equal to 15 μm in the lithium phosphate is greater than 0 and less than or equal to 1%; the proportion of particles with a diameter greater than or equal to 200 nm and less than or equal to 1 μm in the lithium phosphate is greater than or equal to 54% and less than or equal to 70%; and the proportion of particles with a diameter greater than or equal to 50 nm and less than 200 nm in the lithium phosphate is greater than or equal to 30% and less than or equal to 45%. This application, by further controlling the proportion of lithium phosphate particles of three different sizes within the above ranges, not only improves the tap density and compaction density of the lithium phosphate, but also enhances its conductivity and improves the kinetic performance of the lithium secondary battery.

[0011] In any embodiment, the lithium secondary battery further includes a negative electrode sheet, which includes a negative electrode film layer, and the negative electrode film layer includes a negative electrode active material, which includes a silicon-based material and graphite. This application, by adding a silicon-based material to the negative electrode active material, addresses the following: Firstly, due to the intrinsic thermodynamic differences between graphite and silicon, the delithiation voltage of silicon is higher than that of graphite. During the early cycling stages, the negative electrode discharge potential is low, and the lithium ions embedded in the silicon are not completely released but are partially stored. As the cycling process progresses, the negative electrode discharge potential increases, and the lithium ions stored in the silicon are further slowly released back to the positive electrode, thereby improving the problem of excessively rapid degradation of early cycle life in lithium secondary batteries containing lithium phosphate. Secondly, the silicon-based material can also improve the fast-charging capability and energy density of the lithium secondary battery.

[0012] In any embodiment, the mass percentage of silicon in the negative electrode film is greater than 0 and less than or equal to 9 wt%. By ensuring that the mass percentage of silicon in the negative electrode film is within the above range, this application not only helps to improve the problem of excessively rapid degradation of early cycle life in lithium secondary batteries, including those containing lithium phosphate, but also enables the lithium secondary battery to maintain a high initial efficiency and improve its energy density.

[0013] In any embodiment, the mass percentage of silicon in the negative electrode film is 1.5wt% to 6wt%.

[0014] In any embodiment, the capacity of the negative electrode is greater than 370 mAh / g and less than or equal to 700 mAh / g. By ensuring that the capacity of the negative electrode is within the above range, this application not only helps to improve the problem of excessively rapid degradation of the early cycle life of lithium secondary batteries, including those containing lithium phosphate, but also enables the lithium secondary battery to maintain a high initial efficiency level and improve the energy density of the lithium secondary battery.

[0015] In any embodiment, the capacity of the negative electrode is 425mAh / g to 570mAh / g.

[0016] In any embodiment, the silicon-based material includes a silicon-carbon composite material, which comprises a porous carbon matrix and silicon-containing materials distributed within the porous carbon matrix. The silicon-carbon composite material combines the high capacity of silicon with the stability of carbon materials. Through the buffering effect of the porous carbon matrix, improved conductivity, and SEI film stabilization, it effectively alleviates the problems of silicon volume expansion, pulverization, and low conductivity, thereby exhibiting lower volume expansion during charge and discharge and improving the cycle performance of lithium secondary batteries, including those containing lithium phosphate.

[0017] In any embodiment, the compaction density of the lithium phosphate at 3t is 2.58 g / cc to 2.78 g / cc. This application improves the energy density of the lithium secondary battery including the lithium phosphate by ensuring the compaction density of the lithium phosphate at 3t is within the above range, i.e., containing more lithium phosphate per unit volume. Simultaneously, the higher compaction density results in tighter contact between lithium phosphate particles, reducing contact resistance.

[0018] In any embodiment, the tap density of the lithium phosphate is ≥0.8 g / cc. By ensuring that the tap density of the lithium phosphate is within the above range, this application not only facilitates the achievement of high compaction density of the lithium phosphate powder during the compaction process, but also enables closer contact between lithium phosphate particles, reduces contact resistance, and improves the energy density of lithium secondary batteries including lithium phosphate.

[0019] A third aspect of this application provides an electrical device that includes the lithium secondary battery described in the above embodiments, the lithium secondary battery being used to provide electrical energy. Attached Figure Description

[0020] Figure 1 is a schematic diagram of a lithium secondary battery according to an embodiment of this application.

[0021] Figure 2 is an exploded view of a lithium secondary battery according to an embodiment of this application, as shown in Figure 1.

[0022] Figure 3 is a schematic diagram of a battery module according to one embodiment of this application.

[0023] Figure 4 is a schematic diagram of a battery pack according to one embodiment of this application.

[0024] Figure 5 is an exploded view of the battery pack of one embodiment of this application shown in Figure 4.

[0025] Figure 6 is a schematic diagram of an electrical device using a lithium secondary battery as a power source according to an embodiment of this application.

[0026] Explanation of reference numerals in the attached figures:

[0027] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Lithium secondary battery; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Embodiments of the present invention

[0028] The following detailed description, with appropriate reference to the accompanying drawings, discloses a lithium secondary battery and an electrical device according to this application. However, unnecessary details may be omitted. For example, detailed descriptions of well-known facts and repetitive descriptions of essentially identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0029] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0030] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0031] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0032] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0033] When lithium phosphate cathode materials are used as positive electrode active materials and graphite is used as negative electrode active materials to make lithium secondary batteries, the initial charge-discharge efficiency (first efficiency) of lithium phosphate cathode materials is usually higher, while that of graphite is lower. Due to the mismatch of the first efficiency of positive and negative electrodes, lithium ions are lost at the negative electrode, which may lead to the premature degradation of the early cycle life of lithium secondary batteries containing lithium phosphate.

[0034] To address the issue of rapid degradation of early cycle life in lithium-ion batteries containing lithium phosphate, lithium-ion phosphate particles with a particle size ≥1μm are used as the positive electrode active material. However, using lithium-ion phosphate particles with a particle size ≥1μm as the positive electrode active material prevents the compaction density of the positive electrode sheet from being increased, resulting in a lower energy density of the lithium-ion battery.

[0035] Based on this, this application proposes a lithium secondary battery and an electrical device, and the following provides a more detailed description of this application and its optional embodiments.

[0036] This application provides a lithium secondary battery and an electrical device, which includes a positive electrode sheet, a positive electrode active material, and a lithium phosphate-containing material. The positive electrode active material contains lithium phosphate particles with a particle size ≥ 1 μm, accounting for ≥ 0.2% of the total. The lithium phosphate contains particles with a particle size ≥ 5 μm and ≤ 15 μm, accounting for ≥ 0% and ≤ 3% of the total. The lithium phosphate contains particles with a particle size ≥ 200 nm and ≤ 1 μm, accounting for ≥ 50% and ≤ 75% of the total. The lithium phosphate contains particles with a particle size ≥ 50 nm and ≤ 200 nm, accounting for ≥ 25% and ≤ 50% of the total.

[0037] Lithium-containing phosphates refer to a class of phosphates containing lithium ions (Li). + ) and phosphate ions (PO4³ - Lithium phosphates are compounds widely used as cathode materials in lithium-ion batteries. Common lithium-containing phosphates include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate (LiCoPO4), and lithium nickel phosphate (LiNiPO4). Lithium-containing phosphates offer advantages such as high safety, long cycle life, and environmental friendliness as cathode materials, but they also suffer from low energy density.

[0038] The percentage of lithium phosphate particles with a diameter ≥1μm in the positive electrode active material refers to the percentage of lithium phosphate particles with a diameter ≥1μm in the total number of positive electrode active materials.

[0039] The percentage of lithium phosphate particles with a diameter greater than or equal to 5 μm and less than or equal to 15 μm refers to the percentage of lithium phosphate particles with a diameter greater than or equal to 5 μm and less than or equal to 15 μm out of the total number of lithium phosphate particles.

[0040] The percentage of lithium phosphate particles with a diameter greater than or equal to 200 nm and less than or equal to 1 μm refers to the percentage of lithium phosphate particles with a diameter greater than or equal to 5 μm and less than or equal to 15 μm out of the total number of lithium phosphate particles.

[0041] The percentage of lithium phosphate particles with a diameter greater than or equal to 50 nm and less than 200 nm refers to the percentage of lithium phosphate particles with a diameter greater than or equal to 5 μm and less than or equal to 15 μm out of the total number of lithium phosphate particles.

[0042] As an example, the percentage of particles with a diameter greater than or equal to 5 μm and less than or equal to 15 μm in lithium phosphate can be 0.1%, 0.2%, 0.5%, 0.8%, 1%, 1.5%, 2%, 2.5%, or 3%. The percentage of particles with a diameter greater than or equal to 200 nm and less than or equal to 1 μm in lithium phosphate can be 50%, 52%, 55%, 58%, 60%, 62%, 65%, 68%, 70%, 72%, or 75%. The percentage of particles with a diameter greater than or equal to 50 nm and less than 200 nm in lithium phosphate can be 25%, 30%, 35%, 40%, 45%, or 49%.

[0043] The percentages of lithium phosphate particles with a diameter ≥1μm, the percentages of lithium phosphate particles with a diameter greater than or equal to 5μm and less than or equal to 15μm, the percentages of lithium phosphate particles with a diameter greater than or equal to 200nm and less than or equal to 1μm, and the percentages of lithium phosphate particles with a diameter greater than or equal to 50nm and less than 200nm can all be obtained from SEM images. The specific method is as follows: set the magnification of the scanning electron microscope to 5kx, then count the number of particles in each particle size range and the total number of particles in the scanning electron microscope image, and calculate the percentage.

[0044] The positive electrode active material of this application for a lithium secondary battery includes lithium phosphate particles with a particle size ≥1μm. However, these lithium phosphate particles with a particle size ≥1μm not only have low packing density, making it difficult to improve compaction density, but also have a longer diffusion path for lithium ions due to their larger size, resulting in a slower diffusion rate. Consequently, during charging and discharging, lithium ions cannot be rapidly inserted / extracted, leading to a decrease in capacity utilization and energy density. This application, by controlling the ratio of three different particle sizes of lithium phosphate particles within the aforementioned range, achieves better gradation among the three particle sizes. These particles can cooperate and fill each other, thereby improving the tap density and compaction density of the lithium phosphate, and ultimately improving the energy density of the lithium secondary battery containing lithium phosphate.

[0045] In some embodiments, the proportion of lithium phosphate particles with a particle size ≥1μm in the positive electrode active material is 0.2% to 2%.

[0046] As an example, the proportion of lithium phosphate particles with a particle size ≥1μm in the positive electrode active material can be 0.2%, 0.5%, 1%, 1.5% or 2%.

[0047] Optionally, the proportion of lithium phosphate particles with a particle size ≥1μm in the positive electrode active material is 0.2%~10%.

[0048] This application addresses the issue of lithium phosphate particles with a diameter ≥1μm having a proportion within the aforementioned range. Since lithium phosphate particles with a diameter ≥1μm have poor conductivity and thus high polarization, during the initial cycling phase, not all lithium ions can return to the negative electrode after deintercalation. As cycling progresses, the lattice of these particles expands and contracts repeatedly, causing particle breakage and exposing more interfaces. This improves the conductivity of the lithium phosphate particles with a diameter ≥1μm, reduces polarization, and allows more anions to flow back from the negative electrode to the positive electrode, thereby mitigating the problem of rapid early cycle life degradation in lithium secondary batteries containing lithium phosphate.

[0049] In some embodiments, the proportion of particles with a diameter greater than or equal to 5 μm and less than or equal to 15 μm in the lithium phosphate is greater than 0 and less than or equal to 1%, the proportion of particles with a diameter greater than or equal to 200 nm and less than or equal to 1 μm in the lithium phosphate is greater than or equal to 54% and less than or equal to 70%, and the proportion of particles with a diameter greater than or equal to 50 nm and less than 200 nm in the lithium phosphate is greater than or equal to 30% and less than or equal to 45%.

[0050] Among them, lithium phosphate particles with a particle size greater than or equal to 5 μm and less than or equal to 15 μm can be used to improve the tap density of lithium phosphate, lithium phosphate particles with a particle size greater than or equal to 200 nm and less than or equal to 1 μm can improve the compaction density, and lithium phosphate particles with a particle size greater than or equal to 50 nm and less than 200 nm have good conductivity and can improve the kinetic performance of the prepared lithium secondary battery.

[0051] The proportion of lithium phosphate particles of three different sizes in lithium phosphate can be changed by altering the calcination time / temperature, and the roundness can be controlled by adding surfactants such as polyethylene glycol to achieve higher compaction density.

[0052] This application, by further controlling the ratio of three lithium phosphate particles of different sizes within the above-mentioned range, not only improves the tap density and compaction density of lithium phosphate, but also enhances the conductivity of lithium phosphate and improves the kinetic performance of lithium secondary batteries.

[0053] In some embodiments, the lithium secondary battery further includes a negative electrode sheet, which includes a negative electrode film layer, which includes a negative electrode active material, and the negative electrode active material includes silicon-based materials and graphite.

[0054] Silicon-based materials refer to a type of negative electrode active material that includes silicon. Since silicon has a high theoretical specific capacity of about 4200 mAh / g, which is much higher than that of traditional graphite negative electrodes (about 370 mAh / g), silicon-based materials can be used to improve the energy density of lithium secondary batteries and show broad application prospects. However, silicon-based materials have defects such as significant volume expansion, which still need to be improved.

[0055] This application improves the problem of rapid cycle life decay in lithium secondary batteries, including those containing lithium phosphate, by adding silicon-based materials to the negative electrode active material. On the one hand, due to the intrinsic thermodynamic differences between graphite and silicon, the delithiation voltage of silicon is higher than that of graphite. During the early cycle process, the negative electrode discharge potential is low, and the lithium ions embedded in silicon will not be completely released, but will be partially stored. As the cycle process progresses, the negative electrode discharge potential increases, and the lithium ions stored in silicon are further slowly released back to the positive electrode, thereby improving the problem of rapid cycle life decay in the early cycle of lithium secondary batteries, including those containing lithium phosphate. On the other hand, silicon-based materials can also improve the fast charging capability and energy density of lithium secondary batteries.

[0056] In some implementations, the mass percentage of silicon in the negative electrode film is greater than 0 and less than or equal to 9 wt%.

[0057] The mass percentage of silicon in the negative electrode film refers to the percentage of the mass of silicon in the negative electrode film.

[0058] As an example, the mass percentage of silicon in the negative electrode film can be 0.1wt%, 0.5wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, or 9wt%.

[0059] This application, by ensuring that the mass percentage of silicon in the negative electrode film is within the aforementioned range, not only helps to improve the problem of excessively rapid degradation of early cycle life in lithium secondary batteries, including those containing lithium phosphate, but also enables the lithium secondary battery to maintain a high initial efficiency and improve its energy density.

[0060] In some implementations, the silicon content in the negative electrode film is 1.5wt% to 6wt%.

[0061] Optionally, the silicon content in the negative electrode film is 4wt%~5wt%.

[0062] Optionally, the silicon content in the negative electrode film is 4.5 wt%.

[0063] In some implementations, the capacity of the negative electrode is greater than 370 mAh / g and less than or equal to 700 mAh / g.

[0064] The capacity of the negative electrode can be measured by the following methods:

[0065] The negative electrode was prepared as follows: One side of the electrode, which was coated with a film on both sides, was wiped off with alcohol, and then dried in an oven to obtain a single-sided electrode. The single-sided electrode was then cut into circular pieces with a diameter of 14 mm to obtain the negative electrode.

[0066] The electrolyte was prepared as follows: ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 20:20:60. LiPF6 was then uniformly dissolved in the solution, and fluoroethylene carbonate (FEC) was added as an additive to obtain the electrolyte. The concentration of LiPF6 was 1 mol / L, and the mass percentage of FEC in the electrolyte was 5%.

[0067] The coin cell was prepared as follows: using the aforementioned negative electrode as the working electrode, lithium metal as the counter electrode, and a polypropylene film as the separator, the negative electrode, separator, and lithium metal were stacked in sequence, with the separator positioned between the working electrode and the counter electrode. Electrolyte was then injected to assemble the coin cell. The assembled coin cell was placed in a 25°C constant temperature chamber and allowed to stand for 30 minutes. It was then discharged at a constant current of 0.1C to 5mV, followed by a constant voltage discharge to 0.05C to obtain the discharge capacity C1 of the electrode.

[0068] As an example, the capacity of the negative electrode can be 375mAh / g, 390mAh / g, 400mAh / g, 450mAh / g, 500mAh / g, 550mAh / g, 600mAh / g, 650mAh / g, or 700mAh / g.

[0069] This application, by ensuring that the capacity of the negative electrode is within the aforementioned range, not only helps to improve the problem of excessively rapid degradation of the early cycle life of lithium secondary batteries, including those containing lithium phosphate, but also enables the lithium secondary battery to maintain a high initial efficiency level and improve the energy density of the lithium secondary battery.

[0070] In some implementations, the capacity of the negative electrode is 425 mAh / g to 570 mAh / g.

[0071] Optionally, the capacity of the negative electrode is 450mAh / g to 550mAh / g.

[0072] Optionally, the capacity of the negative electrode is 500mAh / g.

[0073] In some embodiments, the silicon-based material includes a silicon-carbon composite material, which includes a porous carbon matrix and silicon-containing materials distributed in the porous carbon matrix.

[0074] Silicon-carbon composite material refers to a new type of composite material that combines silicon (Si) and carbon (C) materials. The carbon material forms a porous structure, and silicon-containing materials are distributed in the porous structure. The porous structure can provide space for the volume expansion of silicon, reduce mechanical stress, and at the same time, the porous structure can increase the contact area between the electrode and the electrolyte, thereby improving the lithium-ion diffusion rate.

[0075] It should be noted that silicon-carbon composite materials can be obtained through chemical vapor deposition. Chemical vapor deposition deposits silane gas into the pores of porous carbon to form silicon-carbon composite materials. After formation and recycling, the pores of the porous carbon may contain a variety of different silicon-containing substances, such as silicon particles.

[0076] Silicon-carbon composite materials combine the high capacity of silicon with the stability of carbon materials. Through the buffering effect of the porous carbon matrix, the improvement of conductivity, and the stabilization of the SEI film, they can effectively alleviate the problems of volume expansion, pulverization, and low conductivity of silicon. As a result, they have lower volume expansion during charge and discharge, which improves the cycle performance of lithium secondary batteries, including those containing lithium phosphate.

[0077] This application does not limit the preparation method of silicon-carbon composite materials, but provides a preparation method of silicon-carbon composite materials as follows:

[0078] Step 1: Weigh 500g of porous carbon and place it in a reactor (rotary kiln or fluidized bed). Heat it to 500℃ under nitrogen protection and keep it at that temperature for 2 hours to allow the air adsorbed inside the porous carbon to be fully desorbed.

[0079] Step 2: A mixture of silane and nitrogen (silane to nitrogen volume ratio 1:4) is introduced into the fluidized bed at a flow rate of 15 L / min, for a total of 360 L of silane, to obtain silicon-carbon particles.

[0080] Step 3: The fluidized bed temperature is raised to 600℃, and a mixture of acetylene and nitrogen gas (acetylene to nitrogen volume ratio 1:4) is introduced at a rate of 15L / min, for a total of 370L of acetylene, to coat the silicon-carbon particles and obtain the intermediate silicon-carbon composite material.

[0081] Step 4: The intermediate silicon-carbon composite material from Step 3 is fed into a rotary kiln for secondary coating. The temperature of the rotary kiln is raised to 600°C, and a mixture of acetylene and nitrogen (acetylene to nitrogen volume ratio of 1:2) is introduced at a rate of 3L / min. A total of 96L of acetylene is introduced to obtain the silicon-carbon composite material.

[0082] In some implementations, the compaction density of lithium phosphate at 3t is 2.58 g / cc to 2.78 g / cc.

[0083] Compacted density refers to the mass per unit volume of a material after pressure is applied. The calculation formula is:

[0084] Compacted density = Mass after compaction / Volume after compaction

[0085] As an example, the compaction density of lithium phosphate at 3t can be 2.58 g / cc, 2.60 g / cc, 2.62 g / cc, 2.65 g / cc, 2.68 g / cc, 2.70 g / cc, 2.72 g / cc, 2.75 g / cc or 2.78 g / cc.

[0086] This application improves the energy density of lithium secondary batteries containing lithium phosphate by increasing the compaction density of lithium phosphate at 3t within the aforementioned range, i.e., containing more lithium phosphate per unit volume, while simultaneously increasing the compaction density to make the lithium phosphate particles more closely connected, reducing contact resistance.

[0087] In some implementations, the tap density of the lithium phosphate is ≥0.8 g / cc.

[0088] Tap density is the mass per unit volume of a powder or granular material after vibration or impact. The formula for calculation is:

[0089] Tap density = mass after vibration / volume after vibration.

[0090] As an example, the tap density of lithium phosphate can be 0.8 g / cc, 0.85 g / cc, 0.9 g / cc, 0.95 g / cc, 1 g / cc, 1.05 g / cc, 1.1 g / cc, 1.15 g / cc or 1.2 g / cc.

[0091] This application, by ensuring that the tap density of lithium phosphate is within the aforementioned range, not only facilitates the achievement of high tap density in the lithium phosphate powder during the compaction process, but also enables closer contact between lithium phosphate particles, reduces contact resistance, and improves the energy density of lithium secondary batteries containing lithium phosphate.

[0092] In addition, the following description, with appropriate reference to the accompanying drawings, will illustrate a lithium secondary battery and an electrical device of this application.

[0093] [Lithium-ion rechargeable battery]

[0094] This application does not impose any particular restrictions on the type of lithium secondary battery; for example, the lithium secondary battery can be a lithium-ion battery, etc.

[0095] Typically, a lithium-ion secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and extracting. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0096] This application does not impose any particular limitation on the type of electrolyte, which can be selected according to actual needs. For example, the electrolyte can be selected from at least one of solid electrolytes and liquid electrolytes (i.e., electrolyte solutions).

[0097] [Positive electrode plate]

[0098] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0099] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0100] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0101] In some embodiments, when the lithium secondary battery is a lithium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for lithium-ion batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials include at least lithium-containing phosphates, and may use only one or a combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, lithium nickel cobalt manganese oxides (such as LiNi1 / 3Co1 / 3Mn1 / 3O2 (also abbreviated as NCM333), LiNi0.5Co0.2Mn0.3O2 (also abbreviated as NCM523), LiNi0.5Co0.25Mn0.25O2 (also abbreviated as NCM211), and LiNi0.6C. At least one of the following: 0.2Mn0.2O2 (also abbreviated as NCM622), LiNi0.8Co0.1Mn0.1O2 (also abbreviated as NCM811), lithium nickel cobalt aluminum oxide (such as LiNi0.8Co0.15Al0.05O2) and their modified compounds. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also abbreviated as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0102] In some embodiments, in order to further increase the energy density of a lithium secondary battery, the positive electrode active material for a lithium ion battery may include one or more of lithium transition metal oxides with the general formula LiaNibCocMdOeAf and modified compounds thereof. 0.8≤a≤1.2, 0.5≤b<1, 0<c<1, 0<d<1, 1≤e≤2, 0≤f≤1, M is selected from one or more of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti and B, and A is selected from one or more of N, F, S and Cl.

[0103] In some embodiments, by way of example, the positive electrode active material for a lithium ion battery may include one or more of LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNi1 / 3Co1 / 3Mn1 / 3O2 (NCM333), LiNi0.5Co0.2Mn0.3O2 (NCM523), LiNi0.6Co0.2Mn0.2O2 (NCM622), LiNi0.8Co0.1Mn0.1O2 (NCM811), LiNi0.85Co0.15Al0.05O2, LiFePO4 and LiMnPO4.

[0104] In the present application, the modified compound of each of the above positive electrode active materials may be doping modification and / or surface coating modification performed on the positive electrode active material.

[0105] As an optional technical solution of the present application, the polyanionic compound may be Li1+xMn1-yAyP1-zRzO4; wherein x is any value within the range of -0.100~0.100, y is any value within the range of 0.001~0.500, z is any value within the range of 0.001~0.100, A includes one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and R includes one or more elements selected from B, S, Si and N;

[0106] As an optional technical approach in this application, the polyanionic compound may be LiaAeMn1-fBfP1-gCgO4-nDn, wherein A includes one or more elements selected from Zn, Al, Na, K, Mg, Nb, Mo, and W; B includes one or more elements selected from Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb, and Ge; C includes one or more elements selected from B, S, Si, and N; D includes one or more elements selected from S, F, Cl, and Br; a is selected from the range of 0.9 to 1.1, e is selected from the range of 0.001 to 0.1, f is selected from the range of 0.001 to 0.5, g is selected from the range of 0.001 to 0.1, n is selected from the range of 0.001 to 0.1, and the second positive electrode active material is electrically neutral.

[0107] During the charging and discharging process of a battery, Li undergoes insertion / extraction and consumption, resulting in varying molar Li content at different discharge states. In the examples of cathode materials in this application, the molar Li content refers to the initial state of the material, i.e., the state before feeding. When the cathode material is applied to the battery system, the molar Li content changes after charge-discharge cycles.

[0108] In the examples of cathode materials in this application, the molar content of O is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of O will fluctuate.

[0109] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0110] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0111] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0112] [Negative electrode plate]

[0113] In some embodiments, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector.

[0114] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0115] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0116] In some embodiments, the negative electrode film layer may optionally include an adhesive. The adhesive may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0117] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0118] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0119] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0120] In other embodiments, the current collector of the negative electrode sheet typically includes a current collector body and a base coating. The base coating can be disposed on at least one side of the current collector body. The base coating basically does not contain negative electrode active material, and may include a small amount of carbon material. However, the carbon material forms a thin coating and cannot function as a negative electrode active material. In this embodiment, the negative electrode sheet can be an electrode sheet without a negative electrode active material layer. For a negative electrode sheet without a negative electrode active material layer, when the current collector of the negative electrode sheet does not contain a base coating, the film layer can be disposed on the surface of at least one side of the current collector; when the current collector of the negative electrode sheet includes a base coating, the film layer can be disposed on the surface of the base coating away from the current collector.

[0121] In some embodiments, the film layer may further include a binder for fixing the additive to the negative electrode sheet. The type of binder is not particularly limited, and those skilled in the art can choose flexibly according to actual needs.

[0122] [Electrolytes]

[0123] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0124] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0125] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0126] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0127] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0128] [Isolation membrane]

[0129] In some embodiments, the lithium secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0130] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0131] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0132] In some embodiments, the lithium secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0133] In some implementations, the outer packaging of the lithium secondary battery can be a hard shell, such as a hard plastic shell, aluminum shell, or steel shell. The outer packaging of the lithium secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0134] This application does not impose any particular limitation on the shape of the lithium secondary battery, which can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square-structured lithium secondary battery 5 as an example.

[0135] In some embodiments, referring to FIG2, the outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to close the receiving cavity. A positive electrode sheet, a negative electrode sheet, and a separator can be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The lithium secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0136] In some implementations, lithium secondary batteries can be assembled into battery modules, and the number of lithium secondary batteries contained in a battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0137] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple lithium secondary batteries 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple lithium secondary batteries 5 can be fixed in place using fasteners.

[0138] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of lithium secondary batteries 5 are received.

[0139] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0140] Figures 4 and 5 show a battery pack 1 as an example. Referring to Figures 4 and 5, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box 2 and a lower box 3, with the upper box 2 covering the lower box 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0141] In addition, this application also provides an electrical device, which includes at least one of the lithium secondary battery, battery module, or battery pack provided in this application. The lithium secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0142] As the electrical device, a lithium secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0143] Figure 6 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the lithium secondary battery for this device, a battery pack or battery module can be used.

[0144] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a lithium-ion battery as their power source.

[0145] Example

[0146] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0147] The relevant parameters of the lithium secondary batteries of Examples 1-12 and Comparative Examples 1-5 are shown in Table 1 below.

[0148] Table 1. Relevant parameters of lithium secondary batteries in Examples 1-12 and Comparative Examples 1-5

[0149]

[0150] The preparation method of lithium iron phosphate in the lithium secondary batteries of Examples 1, 8-12 is as follows:

[0151] Ferric phosphate, lithium carbonate, and glucose were mixed uniformly at a certain molar ratio. The mixture was then ground using a sand mill to obtain large-particle abrasive with a Dv50 particle size of 0.9 μm and small-particle abrasive with a Dv50 particle size of 0.35 μm. The large-particle and small-particle abrasives were mixed at a mass ratio of 7:3. The resulting mixture was sintered at 780℃ under a high-purity nitrogen atmosphere and then naturally cooled. The cooled material was then air-jet pulverized and sintered again at 800℃ under a high-purity nitrogen atmosphere, followed by air-jet pulverization to obtain a lithium transition metal phosphate with a Dv50 of 1.4 ± 0.2 μm and a carbon content of 1.25 ± 0.15% and a carbon coating.

[0152] The preparation method of lithium iron phosphate in the lithium secondary battery of Example 2 is as follows:

[0153] The granular abrasive and the small abrasive particles were mixed at a mass ratio of 6.5:3.5, and the rest of the preparation method was the same as that of lithium iron phosphate in Example 1.

[0154] The preparation method of lithium iron phosphate in the lithium secondary battery of Example 3 is as follows:

[0155] Large and small abrasive particles were mixed at a mass ratio of 6:4, and the rest of the preparation method was the same as that of lithium iron phosphate in Example 1.

[0156] The preparation method of lithium iron phosphate in the lithium secondary battery of Example 4 is as follows:

[0157] Large and small abrasive particles were mixed at a mass ratio of 5.5:4.5, and the rest of the preparation method was the same as that of lithium iron phosphate in Example 1.

[0158] The preparation method of lithium iron phosphate in the lithium secondary battery of Example 5 is as follows:

[0159] The large-particle abrasive Dv50 has a particle size of 0.8 μm, and the rest of the preparation method is the same as that of lithium iron phosphate in Example 1.

[0160] The method for preparing lithium iron phosphate in the lithium secondary battery of Example 6 is as follows:

[0161] The large-particle abrasive Dv50 has a particle size of 1 μm, and the rest is the same as the preparation method of lithium iron phosphate in Example 1.

[0162] The method for preparing lithium iron phosphate in the lithium secondary battery of Example 7 is as follows:

[0163] The large-particle abrasive Dv50 has a particle size of 1 μm and a secondary sintering temperature of 820℃. The rest of the preparation method is the same as that of lithium iron phosphate in Example 1.

[0164] The preparation methods of lithium iron phosphate in the lithium secondary batteries of Comparative Examples 1 and 2 are as follows:

[0165] The large-particle abrasive Dv50 has a particle size of 0.7 μm, and the rest of the preparation method is the same as that of lithium iron phosphate in Example 1.

[0166] The preparation method of lithium iron phosphate in the lithium secondary battery of Comparative Example 3 is as follows:

[0167] The large-particle abrasive Dv50 has a particle size of 2μm, and the rest is the same as the preparation method of lithium iron phosphate in Example 1.

[0168] The preparation method of lithium iron phosphate in the lithium secondary battery of Comparative Example 4 is as follows:

[0169] The small-particle abrasive Dv50 has a particle size of 0.25 μm, and the rest of the preparation method is the same as that of lithium iron phosphate in Example 1.

[0170] The preparation method of lithium iron phosphate in the lithium secondary battery of Comparative Example 5 is as follows:

[0171] The small-particle abrasive Dv50 has a particle size of 0.2 μm, and the rest of the preparation method is the same as that of lithium iron phosphate in Example 1.

[0172] The lithium secondary batteries of Examples 1-11 and Comparative Examples 2-5 of this application and their preparation methods include the following steps:

[0173] S1. Preparation of the positive electrode sheet

[0174] Lithium iron phosphate (LiFePO4), a positive electrode active material, acetylene black, a conductive agent, and polyvinylidene fluoride (PVDF), a binder, were mixed in a weight ratio of 97.9:0.5:1.6 and dissolved in N-methylpyrrolidone (NMP) to prepare a positive electrode slurry. The positive electrode slurry was then uniformly coated onto a 15 μm aluminum foil, achieving a coating areal density of 27 mg / cm². After thorough drying, the slurry was cold-pressed, die-cut, and slit to obtain the positive electrode sheet, which had a compacted density of 2.7 g / cm².

[0175] S2. Preparation of negative electrode sheet

[0176] The negative electrode active material includes a graphite and silicon-carbon composite material. The negative electrode active material, conductive agent carbon black, thickener CMC, binder, and carbon nanotubes are mixed in a weight ratio of 96.4:0.5:1:2:0.1, and deionized water is added. The mixture is stirred evenly under vacuum to obtain a negative electrode slurry. The negative electrode slurry is then coated onto a 6μm copper foil with a coating areal density of 9 mg / cm². After drying, it is cold-pressed, die-cut, and slit to obtain a negative electrode sheet with a compacted density of 1.6 g / cm².

[0177] The silicon-carbon composite material is prepared by the following method:

[0178] Step 1: Weigh 500g of porous carbon and place it in a reactor (rotary kiln or fluidized bed). Heat it to 500℃ under nitrogen protection and keep it at that temperature for 2 hours to allow the air adsorbed inside the porous carbon to be fully desorbed.

[0179] Step 2: A mixture of silane and nitrogen (silane to nitrogen volume ratio 1:4) is introduced into the fluidized bed at a flow rate of 15 L / min, for a total of 360 L of silane, to obtain silicon-carbon particles.

[0180] Step 3: The fluidized bed temperature is raised to 600℃, and a mixture of acetylene and nitrogen gas (acetylene to nitrogen volume ratio 1:4) is introduced at a rate of 15L / min, for a total of 370L of acetylene, to coat the silicon-carbon particles and obtain the intermediate silicon-carbon composite material.

[0181] Step 4: The intermediate silicon-carbon composite material from Step 3 is fed into a rotary kiln for secondary coating. The temperature of the rotary kiln is raised to 600°C, and a mixture of acetylene and nitrogen (acetylene to nitrogen volume ratio of 1:2) is introduced at a rate of 3L / min. A total of 96L of acetylene is introduced to obtain the silicon-carbon composite material.

[0182] S3. Preparation of the isolation membrane

[0183] Using a conventional PE base film with a thickness of 5μm, ceramic fiber, styrene-butadiene rubber, polyvinylidene fluoride and water are mixed evenly in a mass ratio of 4:0.9:0.1:5 to obtain a ceramic fiber slurry. The slurry is then uniformly coated onto the surface of the base film using a gravure coating method to form a ceramic fiber layer, and dried at 45℃ to obtain a separator membrane.

[0184] S4. Preparation of electrolyte

[0185] In an argon atmosphere glove box with a water content of <10ppm, ethylene carbonate (EC), ethyl methyl carbonate, and dimethyl carbonate (DMC) are mixed in a volume ratio of 3:3:4. Then, lithium hexafluorophosphate (LiPF6) is uniformly dissolved in the solvent, and FEC is added to obtain the electrolyte.

[0186] S5, Assembly

[0187] The positive electrode, separator, and negative electrode are prepared in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrode assembly is then wound, hot-pressed and shaped by applying pressure to the flat area, and welded to obtain the electrode assembly. The electrode assembly is placed in a square aluminum shell, vacuum dried, and then injected with electrolyte. After standing, formation testing, aging, and capacity testing, a lithium secondary battery with a final volume of 0.411L is obtained.

[0188] The lithium secondary battery of Example 12 and Comparative Example 1 of this application and its preparation method include the following steps:

[0189] S1. Preparation of the positive electrode sheet

[0190] Lithium iron phosphate (LiFePO4), a positive electrode active material, acetylene black, a conductive agent, and polyvinylidene fluoride (PVDF), a binder, were mixed in a weight ratio of 97.9:0.5:1.6 and dissolved in N-methylpyrrolidone (NMP) to prepare a positive electrode slurry. The positive electrode slurry was then uniformly coated onto a 15 μm aluminum foil, achieving a coating areal density of 27 mg / cm². After thorough drying, the slurry was cold-pressed, die-cut, and slit to obtain the positive electrode sheet, which had a compacted density of 2.7 g / cm².

[0191] S2. Preparation of negative electrode sheet

[0192] The negative electrode active material is graphite. The negative electrode active material, conductive agent carbon black, thickener CMC, binder, and carbon nanotubes are mixed in a weight ratio of 96.4:0.5:1:2:0.1, and deionized water is added. The mixture is stirred evenly under vacuum to obtain a negative electrode slurry. The negative electrode slurry is then coated onto a 6μm copper foil with a coating areal density of 9 mg / cm². After drying, it is cold-pressed, die-cut, and slit to obtain a negative electrode sheet with a compacted density of 1.6 g / cm².

[0193] S3. Preparation of the isolation membrane

[0194] Using a conventional PE base film with a thickness of 5μm, ceramic fiber, styrene-butadiene rubber, polyvinylidene fluoride and water are mixed evenly in a mass ratio of 4:0.9:0.1:5 to obtain a ceramic fiber slurry. The slurry is then uniformly coated onto the surface of the base film using a gravure coating method to form a ceramic fiber layer, and dried at 45℃ to obtain a separator membrane.

[0195] S4. Preparation of electrolyte

[0196] In an argon atmosphere glove box with a water content of <10ppm, ethylene carbonate (EC), ethyl methyl carbonate, and dimethyl carbonate (DMC) are mixed in a volume ratio of 3:3:4. Then, lithium hexafluorophosphate (LiPF6) is uniformly dissolved in the solvent, and FEC is added to obtain the electrolyte.

[0197] S5, Assembly

[0198] The positive electrode, separator, and negative electrode are prepared in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrode assembly is then wound, hot-pressed and shaped by applying pressure to the flat area, and welded to obtain the electrode assembly. The electrode assembly is placed in a square aluminum shell, vacuum dried, and then injected with electrolyte. After standing, formation testing, aging, and capacity testing, a lithium secondary battery with a final volume of 0.411L is obtained.

[0199] In addition, the early cycle performance, first efficiency and energy density of the lithium secondary batteries prepared in Examples 1-12 and Comparative Examples 1-5 were measured, and the results are shown in Table 2.

[0200] The testing method is as follows:

[0201] 1. Early cycle performance

[0202] At 25℃, the lithium secondary battery was left to stand for 30 minutes, then charged at a 0.5C rate to 3.8V, further charged at a constant voltage of 3.8V to a current of 0.05C, left to stand for 5 minutes, and then discharged at a 0.5C rate to a voltage of 2.0V. The resulting capacity was recorded as the initial capacity C0. This constitutes one charge-discharge cycle. The above steps were repeated for the same lithium secondary battery, and the discharge capacity Cn was recorded at each cycle. The battery capacity retention rate after each cycle was Pn = Cn / C0 * 100%, until Pn ≦ 99%, at which point the test was stopped, and the number of cycles was recorded.

[0203] 2. First Coulomb efficiency

[0204] Step 1: At 45℃, let the secondary battery stand for 30 minutes after electrolyte injection, and then charge it at a rate of 0.02C to 30% SOC to form the cell, thus obtaining the charging capacity Q0.

[0205] Step 2: At 25°C, let the formed secondary battery stand for 30 minutes, then charge it at a rate of 0.33C to 3.8V, and further charge it at a constant voltage of 3.8V until the current is 0.05C to obtain the charging capacity Q1.

[0206] Step 3: After the fully charged cell is left to stand for 30 minutes, it is discharged to 2.0V with a current of 0.33C to obtain the discharge capacity Q3.

[0207] Step 4: Calculate the initial coulombic efficiency ICE = Q3 / (Q1 + Q2) * 100%.

[0208] 3. Energy density

[0209] The volumetric energy density of a single battery cell = energy of the single battery cell / volume of the single battery cell.

[0210] The energy of a single battery cell is measured as follows: At 25°C, the battery cell is charged and discharged within a voltage range of 2V-3.8V using a current density of 0.33C. After three cycles, the energy of the third discharge cycle is recorded as the energy of the battery cell. The unit for battery cell energy is Wh, and the unit for battery cell capacity is Ah.

[0211] Table 2. Early cycle performance, first-time efficiency, and energy density of lithium secondary batteries prepared in Examples 1-12 and Comparative Examples 1-5.

[0212]

[0213] As shown in Examples 1-7, when the proportion of particles with a diameter greater than or equal to 5 μm and less than or equal to 15 μm in the lithium phosphate is greater than 0 and less than or equal to 3%, the proportion of particles with a diameter greater than or equal to 200 nm and less than or equal to 1 μm is greater than or equal to 50% and less than or equal to 75%, the proportion of particles with a diameter greater than or equal to 50 nm and less than 200 nm is greater than or equal to 25% and less than 50%, and the proportion of lithium phosphate particles with a diameter ≥ 1 μm in the positive electrode active material is 0.2% to 2%, the energy density of the lithium secondary battery is ≥ 453 Wh / L, the initial coulombic efficiency is ≥ 85%, and the 99% SOC cycle life is ≥ 99%. The number of cycles is ≥300cls. As shown in Examples 1-4, when the proportion of lithium phosphate particles with a particle size ≥1μm in the positive electrode active material is 1%, the energy density of the lithium secondary battery is ≥470Wh / L, the initial coulombic efficiency is ≥88.6%, and the number of cycles at 99% SOC is ≥300cls. As shown in Examples 1, 5-7, when the proportion of lithium phosphate particles with a particle size ≥1μm in the positive electrode active material is 0.2%~2%, the number of cycles at 99% SOC increases with the increase of the proportion of lithium phosphate particles with a particle size ≥1μm in the positive electrode active material, but the initial coulombic efficiency and energy density decrease.

[0214] As can be seen from Examples 1, 8-12, when the silicon content in the negative electrode is 0-6 wt%, the initial coulombic efficiency decreases and the energy density increases with the increase of the silicon content in the negative electrode. The number of 99% SOC cycles first increases and then decreases. When the silicon content in the negative electrode is about 4.5 wt%, the number of 99% SOC cycles reaches its maximum value.

[0215] Comparing Comparative Example 1 and Example 1, it can be seen that the positive electrode active material of Comparative Example 1 contains only 0.1% lithium phosphate particles with a particle size ≥1μm, and the negative electrode does not contain silicon-carbon composite material, but is all graphite. Its 99% SOC cycle count is only 50cls, which is much lower than that of Example 1. The energy density of Comparative Example 1 is only 410Wh / L, which is much lower than that of Example 1.

[0216] As can be seen from the comparison between Comparative Example 2 and Example 1, the proportion of lithium phosphate particles with a particle size ≥1μm in the positive electrode active material of Comparative Example 2 is only 0.1%, and its 99% SOC cycle count is only 150cls, which is much lower than that of Example 1.

[0217] Comparing Comparative Example 3 and Example 1, it can be seen that the proportion of particles with a diameter greater than or equal to 5 μm and less than or equal to 15 μm in the lithium phosphate of Comparative Example 3 is 4%. The first coulombic efficiency of Comparative Example 3 is only 83.2%, which is much lower than that of Example 1. The energy density of Comparative Example 3 is only 430 Wh / L, which is much lower than that of Example 1.

[0218] Comparing Comparative Example 4 and Example 1, it can be seen that the proportion of particles with a diameter greater than or equal to 200 nm and less than or equal to 1 μm in the lithium phosphate of Comparative Example 4 is 80%, and the energy density of Comparative Example 4 is only 429 Wh / L, which is much lower than that of Example 1.

[0219] Comparing Comparative Example 5 and Example 1, it can be seen that the proportion of particles with a diameter greater than or equal to 50 nm and less than 200 nm in the lithium phosphate of Comparative Example 5 is 55%, and the energy density of Comparative Example 5 is only 425 Wh / L, which is much lower than that of Example 1.

[0220] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A lithium secondary battery, wherein, The lithium secondary battery includes a positive electrode sheet, the positive electrode sheet includes a positive electrode active material, the positive electrode active material includes lithium phosphate, and the proportion of lithium phosphate particles with a particle size ≥1μm in the positive electrode active material is ≥0.2%. The proportion of particles with a diameter greater than or equal to 5 μm and less than or equal to 15 μm in the lithium phosphate is greater than 0 and less than or equal to 3%, the proportion of particles with a diameter greater than or equal to 200 nm and less than or equal to 1 μm in the lithium phosphate is greater than or equal to 50% and less than or equal to 75%, and the proportion of particles with a diameter greater than or equal to 50 nm and less than 200 nm in the lithium phosphate is greater than or equal to 25% and less than 50%.

2. The lithium secondary battery according to claim 1, wherein, The proportion of lithium phosphate particles with a particle size ≥1μm in the positive electrode active material is 0.2%~2%.

3. The lithium secondary battery according to claim 1 or 2, wherein, The proportion of particles with a diameter greater than or equal to 5 μm and less than or equal to 15 μm in the lithium phosphate is greater than 0 and less than or equal to 1%, the proportion of particles with a diameter greater than or equal to 200 nm and less than or equal to 1 μm in the lithium phosphate is greater than or equal to 54% and less than or equal to 70%, and the proportion of particles with a diameter greater than or equal to 50 nm and less than 200 nm in the lithium phosphate is greater than or equal to 30% and less than or equal to 45%.

4. The lithium secondary battery according to any one of claims 1 to 3, wherein, The lithium secondary battery further includes a negative electrode sheet, the negative electrode sheet includes a negative electrode film layer, the negative electrode film layer includes a negative electrode active material, and the negative electrode active material includes silicon-based materials and graphite.

5. The lithium secondary battery according to claim 4, wherein, The mass percentage of silicon in the negative electrode film is greater than 0 and less than or equal to 9 wt%.

6. The lithium secondary battery according to claim 4 or 5, wherein, The silicon content in the negative electrode film is 1.5wt% to 6wt%.

7. The lithium secondary battery according to any one of claims 4 to 6, wherein, The capacity of the negative electrode sheet is greater than 370mAh / g and less than or equal to 700mAh / g.

8. The lithium secondary battery according to any one of claims 4 to 7, wherein, The capacity of the negative electrode is 425mAh / g to 570mAh / g.

9. The lithium secondary battery according to any one of claims 4 to 8, wherein, The silicon-based material includes a silicon-carbon composite material, which includes a porous carbon matrix and silicon-containing materials distributed in the porous carbon matrix.

10. The lithium secondary battery according to any one of claims 1 to 9, wherein, The compaction density of the lithium phosphate at 3t is 2.58 g / cc to 2.78 g / cc.

11. The lithium secondary battery according to any one of claims 1 to 10, wherein, The tap density of the lithium phosphate is ≥0.8 g / cc.

12. An electrical appliance, wherein, The electrical device includes a lithium secondary battery as described in any one of claims 1 to 11, wherein the lithium secondary battery is used to provide electrical energy.

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